Assuming that DirecTV supports the DVB-S standard, then the components are as follows:
1. A DVB-S receiver card for your PC with a Common Interface (CI) slot. The CI slot looks a lot like a PCMCIA slot.
2. Software for the receiver card that makes it look like a DVB-T card to MCE.
3. A Common Access Module (CAM). The CAM slots into the CI slot, and provides the interface between the viewing card and the receiver card.
4. Firmware for the CAM that supports the decryption method used by DirecTV.
5. A valid viewing card for DirecTV.
Plug the DVB-S receiver card into the PC, install the software, connect the sat cable to the back of the PC, put the viewing card into the CAM, put the CAM into the CI slot, and start up Media Center. Configure the card, and scan for channels.
The difficult bit is that the EPG and the card type don't match up, so you need to change the headend value in the registry to match the correct EPG and manually match up the listings to the channel (at least that's what I've had to do).
Cheers,
Andrew